This invention relates to the art of shading compensation method in thermal recording using a thermal head.
Thermal recording materials comprising a thermal recording layer on a substrate of a film or the like, which are hereunder referred to as thermal materials, are commonly used to record images produced in diagnosis by ultrasonic scanning.
This recording method, also referred to as thermal recording, eliminates the need for wet processing applied to the image recording which involves the use of silver halide photosensitive materials such as an X-ray film and offers several advantages including convenience in handling. Hence in recent years, the use of the thermal recording is not limited to small-scale applications such as diagnosis by ultrasonic scanning and an extension to those areas of medical diagnoses such as CT, MRI and X-ray photography where large and high-quality images are required is also under review.
As is well known, thermal recording involves the use of a thermal head having a glaze in which heat-generating elements for heating the thermal recording layer of a thermal material to record an image are arranged in one direction (main scanning direction) and, the thermal material or the thermal head is scanned and transported in the auxiliary scanning direction perpendicular to the direction in which the glaze extends, with the glaze a little pressed against the thermal material (thermal recording layer). The thermal head and the thermal material are hence relatively moved in the auxiliary scanning direction and the respective heat-generating elements of the glaze are actuated imagewise by energy application to heat the thermal recording layer of the thermal material, thereby accomplishing image reproduction.
These types of thermal recording have a common problem in that even if it is attempted to perform image recording at uniform density, individual recording apparatuses have their own peculiar characteristics which cause uneven image densities in the main scanning direction. This phenomenon is commonly called "shading" and deteriorates the quality of the recorded image.
For example, the shape of the glaze on the thermal head is not uniform throughout all pixels, but scatters unavoidably; therefore, even if the respective heat-generating elements are supplied with the same amount of energy, they will generate different amounts of heat, causing "shading" or unevenness in the density of the image being recorded.
In order to prevent the deterioration in image quality due to "shading", the thermal recording apparatus having the object of providing high-quality images is adapted to perform "shading compensation", in which the unevenness in image density due to shading is corrected. A typical procedure of shading compensation is as follows. First, image recording is performed on the basis of image data having uniform density in the main scanning direction; the densities of the recorded image are measured and with a certain pixel, say, one of a minimal density, being taken as a reference, the shading compensation data (compensation conditions) which will provide a uniform image density for all pixels are calculated for the respective pixels and shading compensation tables are prepared comprising the respective pixels and shading compensation data. Shading compensation in actual thermal recording is performed by compensating the image data from its supply source by means of the shading compensation data read out of the shading compensation tables.
The inventor has made investigations and found that the shading properties of thermal recording often vary with the recording density and the position on the thermal material in the auxiliary scanning direction.
As described above, thermal recording is performed by heating imagewise the respective heat-generating elements, with the glaze of the thermal head being pressed against the thermal recording layer. Therefore, the temperature of the heat-generating elements varies with the image density, irrespective of whether recording is performed by pulse-width (pulse-number) modulation, or intensity modulation. Thus, the shading properties also vary with the temperature, that is, the recording density.
Thermal recording is performed by scanning the thermal head and the thermal material in the auxiliary direction, as described above. Hence, heat is gradually transmitted from the recording start position toward the end position on a sheet of thermal material to be recorded. In consequence, density gradient is generated between the start and end positions, which gives rise to a variation in the shading properties depending on the recording position in the auxiliary scanning direction.
The conventional shading compensation method as described above can not follow the variation in the shading properties depending on the recording density and recording position. Particularly in applications such as the medical application which require high-quality images, in some cases the thermal recording images having a desired image quality can not be obtained in a consistent manner.